Injection molding is frequently used to make articles for various applications. Because injection molding involves filling a mold with a melted polymer, articles made using injection molding typically suffer some shrinkage when compared to the dimensions of the mold from which they are made. Inadequate mold filling can also result in article surface defects. Shrinkage and surface defects are due, in part, to thermal contraction of the polymer after it has been injected in the mold and allowed to cool. In order to minimize shrinkage and surface defects, it is known to apply higher injection pressures on the polymer stream feeding the mold. This increases cycle times and processing costs.
It is also common to use lower weight molecular polymers to minimize shrinkage and surface defects while using lower injection pressures and/or lower processing temperatures. These lower molecular weight resins include high density polyethylene (HDPE) having a melt flow index (MFI) ranging from 15 to 40, and higher.
Polydiorganosiloxane PPA's are known as potential process additives that can be used in thermoplastics for injection molding applications. These polydiorganosiloxane PPA's are typically fluidic with very low glass transition (Tg) values at room temperature, and flow at room temperature and above without the need for elevated temperatures. Because of their fluidity at ambient conditions, these polydiorganosiloxane PPA's are difficult to handle in injection molding processes. For example, these polydiorganosiloxane PPA's can not be maintained in pellet form at ambient conditions. This makes it difficult to precisely blend these polydiorganosiloxane PPA's with thermoplastics for injection molding applications.
There exists a need for using higher molecular weight polymers in injection molding applications in order to obtain improvements in physical properties, such as flexural strength, elastic modulus, and the like. There also exists a need for using higher molecular weight polymers in injection molding applications while minimizing injection pressures, part shrinkage and the occurrence of surface defects, and reducing cycle times. There also exists a need for a PPA that is easy to handle at ambient conditions and can be used in pellet form for injection molding applications.